Oxford Medical Case Reports, 2016;9, 224–226 doi: 10.1093/omcr/omw064 Case Report

CASE REPORT

When not to trust therapeutic drug monitoring Mathew Westergreen-Thorne1,*†, Sook Yan Lee1,†, Nilesh Shah1 and Alan Dodd2 1

Renal Unit, Renal Admin Block, Kent & Canterbury Hospital, Canterbury, Kent XT3 1RX, UK, and 2Laboratory Medicine, William Harvey Hospital, Ashford, Kent TN24 0LZ, UK

*Correspondence address. Renal Unit, Renal Admin Block, Kent & Canterbury Hospital, Canterbury, Kent XT3 1RX, UK. Tel: +44-7754200758; E-mail: [email protected]

Abstract Therapeutic drug monitoring (TDM) is the measurement of serum or plasma drug concentration to allow the individualization of dosing. We describe the case of a patient who was prescribed inappropriately large doses of vancomycin due to inaccurate TDM. Specifically, our laboratory reported progressively lower vancomycin concentrations despite dose increases. Eventually, when duplicate samples were sent to a different laboratory vancomycin concentrations were found to be in the toxic range. We hypothesize this was due to the patient generating immunoglobulin antibodies against her infection that interfered with the original TDM immunoassay. Immunogenic TDM interference has been known to rarely occur in patients with immune related comorbidities; however, if we are correct, this is a unique case as this patient did not have such a background. This case illustrates the importance of using clinical judgement when interpreting TDM as, in this case, substantial harm to the patient was likely only narrowly avoided.

INTRODUCTION Therapeutic drug monitoring (TDM) has been used since the 1960s to individualize drug therapy. TDM is frequently used for monitoring antibiotics, immunosuppressants and many other medications. However, like all clinical tests, TDM is not infallible. If in sufficiently abnormal quantities, normal serum components can interfere with TDM assays and lead to an improper dose being administered; potentially causing substantial patient harm. Until now, the published literature has only described immunogenic TDM interference in those with immune related comorbidities. Here, we report the rare case of a peritonitis patient without such a background being treated with large and potentially harmful doses of vancomycin from hypothesized TDM interference due to sustained immunoglobulin M (IgM) generation by the patient against their infection. The anomaly

was detected only when the sample was processed at a different laboratory, using a different immunoassay technique not prone to such interference. This case illustrates the importance of having an index of clinical suspicion when interpreting TDM.

CASE REPORT Mrs B was a 71 year old lady established on peritoneal dialysis (PD) due to a history of glomerulo-sclerosis secondary to diabetes mellitus. Her medical background included hypertension and cerebrovascular disease but was without immune related comorbidities. Her most recent PD adequacy and equilibration tests were satisfactory, inferring adequate diffusion of compounds through the peritoneal membrane . She presented to the PD clinic with a purulent discharge from her PD catheter exit site that subsequently grew



Joint first authors. Received: November 7, 2015. Revised: June 16, 2016. Accepted: June 21, 2016 © The Author 2016. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/ licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact [email protected]

224

When not to trust therapeutic drug monitoring

|

225

Table 1: Serum vancomycin concentrations measured using the PETINIA immunoassay at our centre, and the repeat serum vancomycin concentrations performed at an alternative laboratory using the EMIT immunoassay (CRP: C-reactive protein; PETINIA: particle-enhanced turbidimetric inhibition immunoassay; EMIT: enzyme-multiplied immunoassay technique) Day of treatment

WCC (Ref. Range: 4–11 109/L)

Neutrophils (Ref. Range: 2–7.5 109/L)

CRP (mg/dL)

Serum vancomycin concentration (mg/L) Sample timing

PETINIA (Ref. Range: 20–30 mg/L)

EMIT (Ref. Range: 5–10 mg/L)

1

15.4

13.6









4

9.0

6.1

74

Pre-Dose Trough Level

11.2



7

9.2

6.4

60

Pre-Dose Trough Level

17.8



9

9.4

6.2



Pre-Dose Trough Level

3.9



10

10.8

8.2

18

Pre-Dose Trough Level

When not to trust therapeutic drug monitoring.

Therapeutic drug monitoring (TDM) is the measurement of serum or plasma drug concentration to allow the individualization of dosing. We describe the c...
83KB Sizes 1 Downloads 11 Views